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Abstract:

An apparatus including a liquid crystal polymer substrate having a top
surface and a bottom surface, a coplanar waveguide formed on the top
surface of the liquid crystal polymer substrate, the coplanar waveguide
having a 90 degree bend with a mitered edge, an inner via positioned
adjacent to an inner corner of the 90 degree bend, and an outer via
positioned adjacent to the mitered edge of the 90 degree bend, the inner
and outer vias positioned along a first plane that is perpendicular to a
second plane defined by the mitered edge.

Claims:

1. An apparatus comprising:a substrate having a top surface and a bottom
surface;a waveguide formed on the top surface of the substrate, the
waveguide having a 90 degree bend with an inner edge and a chamfered
outer edge; andan inner via being positioned adjacent to the inner edge
of the 90 degree bend.

2. The apparatus of claim 1 wherein the substrate is a liquid crystal
polymer substrate.

3. The apparatus of claim 1 wherein the waveguide is a coplanar waveguide.

4. The apparatus of claim 1 wherein the inner via is positioned about 0.15
millimeters away from the inner edge.

5. The apparatus of claim 1 further comprising an outer via positioned
adjacent to the chamfered outer edge of the 90 degree bend.

6. The apparatus of claim 5 wherein the inner and outer vias are
positioned along a first plane that is perpendicular to a second plane
defined by the chamfered outer edge.

7. The apparatus of claim 5 wherein the chamfered outer via is positioned
about 0.15 millimeters away from the chamfered outer edge.

8. The apparatus of claim 5 wherein the inner and outer vias are filled
with a metal material.

9. The apparatus of claim 5 wherein the inner and outer vias each have a
diameter of about 0.15 millimeters.

10. The apparatus of claim 5 wherein the inner edge, the chamfered outer
edge, the inner via and the outer via are all positioned along a first
plane.

11. An apparatus comprising:a liquid crystal polymer substrate having a
top surface and a bottom surface;a coplanar waveguide formed on the top
surface of the liquid crystal polymer substrate, the coplanar waveguide
having a 90 degree bend with a mitered edge;an inner via positioned
adjacent to an inner corner of the 90 degree bend; andan outer via
positioned adjacent to the mitered edge of the 90 degree bend, the inner
and outer vias being positioned along a first plane that is perpendicular
to a second plane defined by the mitered edge.

12. The apparatus of claim 11 wherein the inner via is positioned about
0.15 millimeters away from the inner corner.

13. The apparatus of claim 11 wherein the outer via is positioned about
0.15 millimeters away from the mitered edge.

14. The apparatus of claim 11 wherein the inner and outer vias are filled
with a metal material.

15. The apparatus of claim 11 wherein the inner and outer vias each have a
diameter of about 0.15 millimeters.

16. The apparatus of claim 11 wherein the inner corner, the mitered edge,
the inner via and the outer via are all positioned along a first plane.

17. An apparatus comprising:a liquid crystal polymer substrate having a
top surface and a bottom surface;a coplanar waveguide formed on the top
surface of the liquid crystal polymer substrate, the coplanar waveguide
having a 90 degree bend with an inner edge and a chamfered outer edge;an
inner via positioned adjacent to the inner edge of the 90 degree bend;
andan outer via positioned adjacent to the chamfered outer edge of the 90
degree bend, the inner and outer vias being positioned along a first
plane that is perpendicular to a second plane defined by the chamfered
outer edge.

18. The apparatus of claim 17 wherein the inner via is positioned about
0.15 millimeters away from the inner edge.

19. The apparatus of claim 17 wherein the outer via is positioned about
0.15 millimeters away from the chamfered outer edge.

20. The apparatus of claim 17 wherein the inner edge, the chamfered outer
edge, the inner via and the outer via are all positioned along a first
plane.

Description:

BACKGROUND

[0001]1. Field

[0002]The invention relates to systems and methods for improving the
performance of 90 degree coplanar waveguide (CPW) bends at mm-wave
frequencies. More particularly, the CPW bends may be chamfered on the
signal conductor and the ground plane and additional vias may be placed
near the CPW bends.

[0003]2. Background

[0004]Microwave and mm-wave RF circuits may be integrated on a dielectric
substrate with transmission lines (e.g., CPW) that feed the RF signals
between the circuits. Such transmission lines often include bends that
turn the direction of energy propagation (i.e., change the direction of
field orientation) from one direction to another. A right angle
transmission line bend, for example, turns the direction of energy
propagation around 90 degrees. One drawback is that transmission line
bends introduce losses.

[0005]One type of loss, called a return loss, relates to the energy that
is reflected back from the transmission line bend. Return losses can be
created due to capacitance and inductance being formed around the
transmission line bends. For example, capacitance may arise through
charge accumulation at the right angle transmission line bend,
particularly, around the outer point of the transmission line bend where
the electric fields concentrate. Inductance may arise due to current flow
constriction. In addition, the change of field orientation at the right
angle transmission line bend is influenced by mode conversions. These
influences significantly increase the return loss.

[0006]Focusing on the return loss, several techniques have been
implemented in the past to compensate for the transmission line bends in
order to reduce the effect of the capacitance and inductance. For
example, the transmission line bends may be mitered and rounded where the
miter technique removes metal where there is no current flow, and that
reduces the capacitance and inductance. Doing so improves the voltage
standing wave ratio (VSWR) and reduces the return loss.

[0007]A coplanar waveguide (CPW) is an attractive choice for the
development of monolithic microwave integrated circuits (MMICs). A CPW is
formed from a conductor separated from a pair of ground planes, all on
the same plane, atop a dielectric medium. Several advantages of CPWs
include ease of shunt and series connections, low radiation, low
dispersion, and avoidance of the need for thin fragile substrates. One
drawback of a prior art CPW bend is that the two slots and the two ground
planes on each side of the center conductor have different lengths. The
different lengths cause unwanted slot-line and parallel plate modes which
tend to radiate and reduce the overall performance of the transmission
line.

[0008]FIG. 1A is a schematic view of a prior art CPW bend 104 that
utilizes air-bridges 102 for performance improvements. FIG. 1B is a
schematic view of a prior art chamfered CPW bend 106 that utilizes
air-bridges 102 for performance improvements. Referring to FIGS. 1A and
1B, the placement of air-bridges 102 near the CPW bends 104 has been used
to eliminate unwanted slot-line and parallel plate modes. However, the
inclusion of air-bridges 102 may add unwanted capacitance on the
transmission lines which can further degrade the CPW performance. CPW
performance is especially important at mm-wave frequencies.

[0009]FIG. 2 is a schematic view of a prior art CPW bend that utilizes
high-impedance transmission line sections 204 under the air-bridges 202
for performance improvements. The high-impedance transmission line
sections 204 under the air-bridges 202 are narrower and therefore add
less parasitic capacitance on the transmission line. However, the
high-impedance transmission line sections 204 require the addition of
short matching networks.

[0010]Although the foregoing techniques are helpful in reducing the return
loss for the transmission line bends, additional improvements can be made
to improve the VSWR and reduce the return loss. Moreover, they require
the fabrication of air-bridges which is complex. Therefore, a need exists
in the art for systems and methods for improving the performance of CPW
bends at mm-wave frequencies without the need for air-bridges.

SUMMARY

[0011]An apparatus including a liquid crystal polymer substrate having a
top surface and a bottom surface, a coplanar waveguide formed on the top
surface of the liquid crystal polymer substrate, the coplanar waveguide
having a 90 degree bend with a mitered edge, an inner via positioned
adjacent to an inner corner of the 90 degree bend, and an outer via
positioned adjacent to the mitered edge of the 90 degree bend, the inner
and outer vias positioned along a first plane that is perpendicular to a
second plane defined by the mitered edge.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]The features, objects, and advantages of the invention will become
more apparent from the detailed description set forth below when taken in
conjunction with the drawings, wherein:

[0013]FIG. 1A is a schematic view of a prior art CPW bend that utilizes
air-bridges for performance improvements;

[0014]FIG. 1B is a schematic view of a prior art chamfered CPW bend that
utilizes air-bridges for performance improvements;

[0015]FIG. 2 is a schematic view of a prior art CPW bend that utilizes
high-impedance transmission line sections under the air-bridges for
performance improvements;

[0016]FIG. 3 is a schematic top view of a three-dimensional automotive
radar RF front-end according to an embodiment of the invention;

[0017]FIG. 4 is a schematic bottom view of the three-dimensional
automotive radar RF front-end according to an embodiment of the
invention;

[0018]FIG. 5 is a schematic diagram showing a back-to-back 90 degree CPW
bends on a LCP according to an embodiment of the invention;

[0019]FIG. 6 is a graph of return loss for the apparatus shown in FIG. 5
according to an embodiment of the invention; and

[0020]FIG. 7 is a graph of insertion loss for the apparatus shown in FIG.
5 according to an embodiment of the invention.

DETAILED DESCRIPTION

[0021]Apparatus, systems and methods that implement the embodiments of the
various features of the invention will now be described with reference to
the drawings. The drawings and the associated descriptions are provided
to illustrate some embodiments of the invention and not to limit the
scope of the invention. Throughout the drawings, reference numbers are
re-used to indicate correspondence between referenced elements.

[0022]FIG. 3 is a schematic top view of a three-dimensional automotive
radar RF front-end 300 having a plurality of CPW bends 305 according to
an embodiment of the invention. FIG. 4 is a schematic bottom view of the
three-dimensional automotive radar RF front-end 300 having a plurality of
CPW bends 305 according to an embodiment of the invention. The plurality
of CPW bends 305 achieve optimum performance by exploiting the
capabilities provided by the use of a liquid crystal polymer (LCP)
substrate. The plurality of CPW bends 305 are wideband (e.g., 60-90 GHz)
to increase fabrication and assembly tolerances and have low-insertion
loss (e.g., less than -1 dB) to reduce the loss between the antenna and
the T/R module. The loss generally dominates the overall noise figure of
the radar and eventually limits its sensitivity and read range. The low
return loss, small size to allow for the co-location of multiple
transitions in close proximity to the chip, low cost, and minimum number
of vias are compatible with LCP design rules.

[0023]The automotive radar RF front-end 300 achieves very good RF
performance (i.e., low insertion and return loss and wide bandwidth) by
utilizing a chamfered or mitered bend on the signal conductor and the
ground plane and by strategically placing vias adjacent to the CPW bends
305. The automotive radar RF front-end 300 may be implemented using
hardware, software, firmware, middleware, microcode, or any combination
thereof. One or more elements can be rearranged and/or combined, and
other radars can be used in place of the radar RF front-end 300 while
still maintaining the spirit and scope of the invention. Elements may be
added to the radar RF front-end 300 and removed from the radar RF
front-end 300 while still maintaining the spirit and scope of the
invention.

[0024]FIG. 5 is a schematic diagram showing a back-to-back 90 degree CPW
bends 505 on a LCP 510 according to an embodiment of the invention. The
CPW bends 505 may be used with automotive radar and passive mm-wave
imager applications. Each CPW bend 505 has a mitered corner 506. In one
embodiment, the signal plane 507 and the ground plane 508 are chamfered.
The CPW bends 505 are formed on a LCP 510 or any other low-cost organic
substrate. The CPW bends 505 are made of a metallic material and are used
to propagate signals.

[0025]An inner via 515 and an outer via 520 are positioned adjacent to
each CPW bend 505. The inner via 515 and the outer via 520 are preferably
metalized vias. The inner via 515 and the outer via 520 lie along a first
plane 521 that is perpendicular to a second plane 522 defined by the CPW
bend 505. The inner via 515 and the outer via 520 enhance and optimize
the performance of the CPW bend 505. The enhancement and optimization of
performance is shown in FIGS. 6 and 7. That is, the return and insertion
losses are reduced from 605, 705 (no chamfering and no vias) to 610, 710
(chamfering and no vias) to 615, 715 (chamfering and vias). In one
embodiment, the inner via 515 is about 0.15 millimeters in diameter,
about 0.1 millimeters in depth, and positioned about 0.15 millimeters
away from the CPW bend 505. Similarly, the outer via 520 is about 0.15
millimeters in diameter, about 0.1 millimeters in depth, and positioned
about 0.15 millimeters away from the CPW bend 505. The inner via 515 and
the outer via 520 are used to suppress the parasitic parallel plate and
slot-line modes. The inner and outer vias 515 and 520 are created with
either laser or mechanical drilling and are filled with metal.

[0026]FIG. 6 is a graph of return loss for the apparatus shown in FIG. 5
according to an embodiment of the invention. Curve 605 shows the return
loss of a bend with a sharp corner, curve 610 shows the return loss of a
bend with a chamfered end, and curve 615 shows the return loss of a bend
with a chamfered end and inner and outer vias 515 and 520. The bend with
a sharp corner (curve 605) can be used for frequencies up to 60 GHz, the
bend with a chamfered end (curve 610) can be used for frequencies up to
75 GHz, and the bend with a chamfered end and inner and outer vias 515
and 520 (curve 615) can be used for frequencies up to 90 GHz and even
higher. Hence, the performance (i.e., return loss) of the bend with a
chamfered end and inner and outer vias 515 and 520 is better than other
bends without the inner and outer vias 515 and 520.

[0027]FIG. 7 is a graph of insertion loss for the apparatus shown in FIG.
5 according to an embodiment of the invention. Curve 705 shows the
insertion loss of a bend with a sharp corner, curve 710 shows the
insertion loss of a bend with a chamfered end, and curve 715 shows the
insertion loss of a bend with a chamfered end and inner and outer vias
515 and 520. The bend with a sharp corner (curve 705) can be used for
frequencies up to 60 GHz, the bend with a chamfered end (curve 710) can
be used for frequencies up to 75 GHz, and the bend with a chamfered end
and inner and outer vias 515 and 520 (curve 715) can be used for
frequencies up to 90 GHz and even higher. Hence, the performance (i.e.,
insertion loss) of the bend with a chamfered end and inner and outer vias
515 and 520 is better than other bends without the inner and outer vias
515 and 520.

[0028]Those of ordinary skill would appreciate that the various
illustrative logical blocks, modules, and algorithm steps described in
connection with the examples disclosed herein may be implemented as
electronic hardware, computer software, or combinations of both. To
clearly illustrate this interchangeability of hardware and software,
various illustrative components, blocks, modules, circuits, and steps
have been described above generally in terms of their functionality.
Whether such functionality is implemented as hardware or software depends
upon the particular application and design constraints imposed on the
overall system. Skilled artisans may implement the described
functionality in varying ways for each particular application, but such
implementation decisions should not be interpreted as causing a departure
from the scope of the disclosed apparatus and methods.

[0029]The previous description of the disclosed examples is provided to
enable any person of ordinary skill in the art to make or use the
disclosed methods and apparatus. Various modifications to these examples
will be readily apparent to those skilled in the art, and the principles
defined herein may be applied to other examples without departing from
the spirit or scope of the disclosed method and apparatus. The described
embodiments are to be considered in all respects only as illustrative and
not restrictive and the scope of the invention is, therefore, indicated
by the appended claims rather than by the foregoing description. All
changes which come within the meaning and range of equivalency of the
claims are to be embraced within their scope.